The human body requires three macronutrients--protein, carbohydrates and fat--to maintain normal function. Glucose falls into the carbohydrate category and in fact is a constituent of table sugar. Starch, also, is made up of pure glucose, despite the fact that it doesn't taste sweet. Glucose uptake by cells depends on the hormone insulin.
Function
The purpose of glucose in the human body is to provide cells with energy. Glucose consumed as sugar or starch is broken down into individual sugar molecules and absorbed into the bloodstream, explains Dr. Lauralee Sherwood in her book "Human Physiology." While glucose in the blood--sometimes called blood sugar--is important because it ensures that all body cells have access to glucose, the body regulates blood sugar so that it gets neither too high nor too low. Insulin is part of that regulation process.
Features
Glucose from the bloodstream needs to enter body cells so that cells can burn the glucose for energy. Some cells--specifically the cells of the liver and muscles--also store glucose in the form of a molecule called glycogen. Cells can't take up glucose on their own; instead, they need insulin to bind to cellular receptors to signal them to take up the glucose from the blood. Insulin is, explains Sherwood, a "gatekeeper" molecule that allows glucose into cells.
Considerations
Under normal circumstances, blood glucose levels rise very slowly after a meal. This is because fiber, fat and protein in food prevent the digestive tract from taking up large quantities of glucose at once. Sometimes, however, meals that are very high in glucose and low in other nutrients can cause blood sugar levels to increase dramatically. This leads to a dramatic insulin release, notes Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." The result of the insulin release in this case is paradoxical hypoglycemia, or low blood sugar, because insulin causes cells to take up blood glucose.
Relationship in Diabetics
In diabetics, the ability of cells to take up blood glucose is severely impaired. Type 1 diabetics, notes Sherwood, don't produce insulin. Without insulin, their blood glucose can rise very high after meals. With insulin injections, however, type 1 diabetics function normally, and cells take up glucose. Type 2 diabetics do produce insulin, but their cells can't respond. As such, they can't take insulin injections to cause cells to take up glucose and instead must limit glucose in the diet.
Expert Insight
The strength of an insulin signal varies significantly with the rate at which glucose enters the bloodstream in an individual with no underlying diseases. If blood sugar, and therefore insulin levels, rises quickly, cells interpret this as a signal to store blood glucose as fat. As such, insulin can actually direct how glucose is used by cells. Slower insulin release leads to utilization of glucose or storage as glycogen, while faster insulin release increases fat storage.
References
- "Human Physiology"; Lauralee Sherwood; 2004
- "Biochemistry"; Reginald Garrett and Charles Grisham; 2007
